The Effects of Road Transport on Freshwater and Marine Ecosystems

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Note: Sources refer Table 5.2.
For PAHs in detention pond sediments, the lower sediment quality guidelines are typically exceeded (e.g.,
the TEL and ER-L).
6.4.3 Stream, wetlands and Lake Sediments
Stormwater within urban areas is typically routed to the closest water body which is typically a stream or
channel in heavily urbanised areas. <strong>The</strong>re are a number <strong>of</strong> New Zealand urban centres which contain
urban lakes. In Hamilton, there is Hamilton Lake; in North Shore City, Lake Pupuke and in Auckland City,
Western Springs. Several cities are located on the shores <strong>of</strong> larger lakes such as Taupo on Lake Taupo;
Rotorua on Lake Rotorua and Queenstown on Lake Wanaka. Freshwater wetlands on streams in urban
areas will act in a similar fashion to a detention pond.
Sediment quality data collected adjacent to an outfall to Hamilton Lake revealed elevated concentrations
<strong>of</strong> Pb and Zn in particular that exceeded ER-L and ER-M sediment quality guidelines (Rajendram 1992 in
Snelder & Trueman 1995). <strong>The</strong>re have been a number <strong>of</strong> studies <strong>of</strong> sediment quality in urban streams in
New Zealand. Data has also been collected on urban stream quality in most large urban areas. <strong>The</strong>re is
considerable data for Auckland Streams such as those in Waitakere City (e.g., Kingett Mitchell 2000).
<strong>The</strong> results <strong>of</strong> that study showed that the common transport related contaminants (Cu, Pb and Zn)
exceeded the ER-L in a high proportion <strong>of</strong> urban catchment samples. Fig. 6.2 illustrates the variation in
key trace element concentrations in catchments with varying landuse. <strong>The</strong> catchments at the top <strong>of</strong> the
figure tend to be rural with the catchments in the lower part <strong>of</strong> the figure completely urban.
Titirangi/Langholm is a bush urban catchment (Kingett Mitchell 2000.
Accumulation <strong>of</strong> contaminants in lakes is a function <strong>of</strong> the local physical characteristics <strong>of</strong> the lake and
lake-bed near the discharge. Macaskill et al. (2003) identified low concentrations <strong>of</strong> key contaminants
near stormwater outfalls in Lake Rotorua.
Yousef et al. (1984) assessed the fate <strong>of</strong> metals discharged from highway bridges in Florida to Lake
Ivanhoe (Maitland interchange). Differences in the concentration <strong>of</strong> a number <strong>of</strong> elements (Zn, Pb, Cr, Ni,
Cu, Fe) were identified below bridge discharge points compared to normal lake concentrations. Gjessing
et al. (1984) also examined the effect <strong>of</strong> highway run<strong>of</strong>f on Lake Padderudvann in Norway.
Concentrations <strong>of</strong> Pb, Zn and Cd were elevated and exceeded guidelines such as the ER-L and TEL and
in the case <strong>of</strong> lead the PEL.
Given the concentration <strong>of</strong> Cu, Pb and Zn in the suspended sediments <strong>of</strong> stormwater draining roads,
exceedence <strong>of</strong> sediment quality guidelines would be expected in areas where stormwater particulates are
deposited. Depending upon the ratio <strong>of</strong> stormwater sediment to uncontaminated stream sediments,
elements such as Pb and Zn may exceed guidelines at which adverse effects (on benthic freshwater
organisms) could be expected to occur. In urban areas however, streams may not receive their normal
supply <strong>of</strong> sediments and as a result sediments derived from roadways and other urbanised sources may
dominate the sediments in the stream. Trace element concentrations in urban streams are approximately
related to the amount <strong>of</strong> impervious cover and time since urbanisation within the catchment
For lakes, the sediment contaminant concentration will depend on the distribution and build-up <strong>of</strong>
contaminants in the lake-bed following stormwater discharge to the lake. Contaminant build-up in small
lakes with low natural sediment budgets would be expected. Available data indicates that contaminant
concentrations in lake sediment adjacent to stormwater discharges from roads can approach and exceed
sediment quality guidelines.
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